docs: def attach(cls, ip, vm, background=False, force=False):
          """ Attach """
          vm_ = Iaas.info(vm)
          ip_ = cls.info(ip)
          if not cls._check_and_detach(ip_, vm_):
              return
  
          # then we should attach the ip to the vm
          attach = Iface._attach(ip_['iface_id'], vm_['id'])
          if not background:
              cls.display_progress(attach)
  
          return attach